The present invention relates to a method for digital transmission in a digital transmission system given packet-switched service wherein for purposes of channel coding a turbo-coding is performed in a turbo-coder at the sender side and a turbo decoding is performed in a turbo-decoder at the receiver side with soft decision output signals.
P. Jung""s xe2x80x9cComparison of Turbo-Code Decoders Applied to Short Frame Transmission Systemsxe2x80x9d (IEEE Journal on Selected Areas in Communications, Vol. 14 (1996):530-537) examines the application of turbo-codes for digital transmission systems, wherein both coders and decoders for the turbo-codes in the transmission path are examined. The decoding of the turbo-codes is based on the use of soft-input/soft-output decoders, which can be perform either using MAP (Maximum a-posteriori) symbol estimators or using MAP sequence estimators, for instance using an estimator having an a priori soft output Viterbi algorithm (APRI-SOVA). In this publication, four different decoder arrangements and their abilities to process defined error rates are described. In addition, the performance of these decoders in different instances of application is examined. It is established that turbo-codes and their iterative decoding are [sic] an effective measure against packet errors.
In xe2x80x9cTurbo-codes for BCS Applicationsxe2x80x9d (D. Divsalar and F. Pollara, ICC ""95, Seattle, Wash., Jun. 18-22, 1995), turbo-codes are suggested in order to achieve error correction nearly reaching what is known as the Shannon limit. For this purpose, relatively simple component codes and large interleavers should be used. In this publication,the turbo-codes are generated in a coder having a plurality of codes and are decoded in a suitable decoder. The turbo-codes were introduced by Berrou et al in 1993 (see C. Berrou, A. Glavieux and P. Thitimayshima, xe2x80x9cNear Shannon Limit Area Correction Coding; Turbo-codesxe2x80x9d, Proc. 1993 IEE International Conference on Communications: 1064-1070). A rather effective error correction can be achieved with this method.
xe2x80x9cIterative Correction of Intersymbol Interference: Turbo-Equalizationxe2x80x9d (Catherine Douillard, ETT European Transactions on Telecommunications, Vol. 6, No. 5, September-October 1995) teaches what is known as turbo-equalization, with which it is intended to eliminate the adverse effects of the intersymbol interference in digital transmission systems which are protected by convolution codes. The receiver executes two consecutive soft output decisions, which are carried out by a symbol detector and a channel decoder via an iterative process. In each iteration, extrinsic information from the detector and the decoder is used in the next iteration and in the turbo-decoding [sic]. It is demonstrated that intersymbol interference effects in multi-path channels can be overcome with turbo-equalization.
Methods for linking turbo-coding, ARQ (Automatic Repeat Request) and radio systems are taught by Narayanan, K. R greater than (xe2x80x9cA Novel ARQ Technique Using the Turbo-Coding Principle; IEEE Communications Letters, vol. 1, Nr. 2, March 1997:49-51) and by Naddhan, M. (xe2x80x9cNew Results on the Application of Antenna Diversity and Turbo-codes in a JD-CDMA Mobile Radio System; 5th IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC ""92) and ICCC Regional Meeting on Wireless Computer Networks (WCN), The Hague, Netherlands, vol. 2, Sept. 18-23, 1994:524-528).
Future transmission systems such as the European UMTS (Universal Mobile Telecommunications System) require the support of a number of coexisting carrier services having carrier data rates of up to 2 Mbit/s in a flexible manner, whereby the best possible spectral efficiency is desired. In ACTS (Advanced Communications Technologies and Services) Project AC090 FRAMES (Future Radio widebAnd Multiple accEss Systems), an MA (Multiple Access) schema called FRAMES Multiple Access (FMA) was developed that satisfies the requirements of UMTS. As a transmission system of the third generation, which includes a wide range of fields of application, carrier services and various scenarios, FMA must satisfy [sic] present and future developments of UMTS radio interface standards. FMA includes two operating modes, namely WB-TDMA (Wideband Time Division Multiple Access) with and without spreading and compatibility with GSM (Global System for Mobile Communications), and WB-CDMA (Wideband Code Division Multiple Access). Although the system considered here is basically according to FMA, it also is possible to include other transmission systems having multiple access methods, such as FDMA (Frequency Division Multiple Access) or MC-CDMA (Multicarrier CDMA) or combinations of these transmission systems.
In view of the high performance of turbo-codes, it is desirable to use them in digital transmission systems. But the diverse requirements in FMA, for instance, make it necessary to take care when such turbo-codes are used that the data transmission is not loaded too heavily by the transmission of error correction codes.
It is an object of the present invention, to make available the a [sic] method for packet transmission with an ARQ protocol on transmission channels in a digital transmission system wherein a turbo-coding is used for channel coding, wherein is possible to keep the channel load due to ARQ optimally small by means of a new turbo-code and a dotting that is tuned thereto.
The above-described method is inventively achieved wherein in order to trigger an ARQ:
the channel quality is estimated by a parameter estimation method that is known per se,
the variances of the soft decision output signals at the turbo-decoder are determined,
the correctness or, respectively, defectiveness of the transmitted packet is inferred from the channel quality and the variances, and
a retransmission of at least a part of a defective packet is triggered.
Previously, an ARQ was triggered in that the data packet to be sent was protected by a suitable error-detecting coding such as CRC (Cyclic Redundancy Check). Defective data blocks then could be detected in the receiver by this type of coding and could be re-requested. A disadvantage of this method is that the error-detecting coding generates additional redundancy bits with which it is not possible to transmit information. In contrast, the inventive method possesses the advantage that the estimation of the quality of service can be applied for purposes of triggering the ARQ. As is laid out in A detail below, for decoded block it is possible to make an unambiguous statement about the correctness of the block, or, respectively, of the packet dependent on the channel quality with the aid of the computed variance, for instance, of the LLRs. One of the known parameter estimating methods, such as MMSE (Minimum Means Square Error Estimation), MOM (Method of Moments) or MVU (Minimum Variance Unbiased Estimation) is used to estimate the channel qualities for this purpose. The error-detecting coding of the data packet is, thus, inventively omitted. This increases both the overall coding rate on the transmission channel and the share of the payload information contained in the packet as well. In addition, the computing outlay for the decoding is reduced. As such, the delay that is generated by the decoding of the error detecting portion is reduced as well.
The term xe2x80x9cquality of servicexe2x80x9d is used here in the following sense: For different services, defined QoS criteria (QoS=Quality of Service ) apply, and the definitions of the QoS criteria for different carrier services have been worked out in the context of FRAMES. An important component of a QoS criterion is the carrier data rate R. The QoS criterion also includes maximum allowable error rate PbG or a packet loss rate PlG in combination with a maximum probability of failure PoutG. In the case of power-switched services, the probability P{Pb greater than PbG} that the instantaneous bit error rate Pb exceeds PbG must not be greater than PoutG, that is
Pr {Pb greater than PbG} less than PoutG.
In voice transmission PbG equals 10xe2x88x923 and PoutG equals 0.05. In packet services a similar condition holds for the instantaneous packet loss rate Pl:
Pr {Pl greater than PlG} less than PoutG.
Besides the criteria relating to Pr, there are additional conditions in the context of the QoS criterion. Mainly the QoS parameters PbG, PlG and PoutG that are directly related to the selection of the error correction code (ECC) are considered here, however. In ECC, the coding rate Rc is essentially determined by the multiplex method, the modulation and the packet parameters. In other words, the coding rate Rc has a direct connection to the question of whether or not a QoS criterion for a specific service is satisfied.
In a method in which a soft input/soft output symbol estimator or sequence estimator is used at the receiver side, it is advantageous when the quality of service is determined from the variances "sgr"2 of the soft decision output signals of the turbo-decoder, wherein the bit error rates are computed from the variances "sgr"2 as a measure of the quality of service.
In a method in which an MAP symbol estimator or an MAP sequence estimator is used at the receiver side, it is advantageous when the quality of service is determined from the variances "sgr"2LLR of the soft decision output signals of the turbo-decoder.
In a method in which a Viterbi algorithm is used for sequence estimation at the receiver side, it is advantageous when the quality of service is determined from the variances "sgr"2VIT of the soft decision output signals of the turbo decoder.
Since the inventive method can be used either given MAP estimators or given an estimation by a Viterbi algorithm, there is practically no restriction with respect to the most important method of sequence and symbol estimation. This is true, although in Th the specific description below, this statement is only substantiated in the context of an MAP symbol estimator.
According to an advantageous development of the present invention, the method is characterized in that what is known as Berrou""s dotting is used for the dotting, in which only the non-systematic information is dotted. This type of dotting is advantageous given lower values of the signal/noise ratio.
According to a further development, the inventive method is characterized in that what is known as UKL dotting is used for the dotting, in which both the systematic information and the non-systematic information are dotted. This type of dotting is advantageous given higher signal/noise ratios and thus given bit rates of  less than 10xe2x88x924.
In another embodiment, the inventive method is characterized in that an RCPTC is used as turbo-code. As can be seen in the detailed description, a code of this type is particularly flexible and is suited to the objects of the present invention. The RCPTC enables the transition from fixed ECC strategies to flexible ECC strategies, the latter being adapted to requirements which vary over time, so that a flexible link layer control and medium access control are possible.
Pursuant to a further developement the inventive method is characterized in that in a retransmission of the information of a defective packet at least a part of the information suppressed by the dotting of the RCPTC in the previous transmission is sent, and that this additional information is inserted into the already existing information at the receiver side, and this complete information is decoded again. In this way, the channel is loaded as little as possible in the use of an ARQ method.
In yet another embodiment, the inventive method is characterized in that in the retransmission only those bits are sent which are additionally available at the next lowest coding rate, since they are not dotted. In this way, only the minimum information required in order to achieve the object of an adequate transmission quality is transmitted.
Moreover, another development of the inventive method is characterized in that the method is repeated until an errorless decoding of the packet has been achieved or until the all the coded information of a packet has been transmitted. In this way, the entire potential of the coding is exhausted in order to correct the errors in a packet.